Most industrial waters contain alkaline earth and transition metal cations such as calcium, barium, iron, zinc, manganese, and magnesium, and several anions such as bicarbonate, carbonate, sulfate, phosphate, silicate, and fluoride. When combinations of these anions and cations are present in concentrations which exceed the solubility of their reaction products, precipitates form until these product solubility concentrations are no longer exceeded. For example, when the ionic product of calcium and carbonate exceeds the solubility of calcium carbonate, a solid phase of calcium carbonate will form.
Solubility product concentrations are exceeded for reasons such as partial evaporation of the water phase, change in pH, pressure or temperature, and the introduction of additional ions which form insoluble compounds with the ions already present in the solution.
As these reaction products precipitate on surfaces of the water-carrying system, they form scale or deposits. This accumulation prevents effective heat transfer, interferes with fluid flow, facilitates corrosive processes, and harbors bacteria. This scale is an expensive problem in many industrial water systems, such as recirculating cooling water systems in cooling towers, in that the scale causes delays and shutdowns for cleaning and removal of the scale.
Citric acid, gluconic acid, and other materials have been used in the past to stabilize iron in solution and thus prevent its precipitation in forms such as ferric hydroxide and ferric oxide, wherein iron is in the third oxidation state. Citric acid and such materials effectively maintain iron in solution by forming complexes therewith which are soluble in water and thereby, remain dissolved in water. Although citric acid and cognate materials are effective as stabilizing agents for iron in solution, they are not antiscalants and are ineffective against scale such as calcium carbonate, calcium phosphate, and calcium sulfate.
In U.S. Pat. No. 3,928,196, a method is disclosed for inhibiting scale formation using two-component copolymers. The copolymers, which contain 5-50 mole percent acrylamidoalkane sulfonic acid and 95-50 mole percent of acrylic acid or a similar acid, have molecular weight of 1,000 to 10,000. The 25/75 mole percent copolymer of acrylamidomethylpropane sulfonic acid (AMPS) and acrylic acid (AA) is equivalent to 49/51 weight ratio whereas the 75/25 mole percent copolymer of AMPS and AA, is equivalent to 89.5/10.5 weight ratio.
The Amick U.S. Pat. No. 4,711,725 discloses stabilization of an aqueous system by inhibiting precipitation of scale with a copolymer antiscalant comprising 42 to 84% of (meth)acrylic acid and salts thereof, 11 to 40% acrylamidoalkane sulfonic acid, and 5 to 30% of one or more monomers selected from vinyl esters, vinyl acetate, and substituted acrylamide. The terpolymers of the Amick patent, when added to an aqueous system in a small amount, exhibit unexpectedly improved precipitation inhibition and dispersion performance for phosphates, iron, zinc, and inorganic particulates. The aqueous medium can contain or can be devoid of iron contamination.
U.S. Pat. No. 4,707,271 discloses the use of copolymers of acrylic acid and a substituted acrylamide as antiscalants in presence of soluble iron in solution. Up to about 30 weight parts, per 100 weight parts of all polymerized monomers, of other monomers can be polymerized with acrylic acid and substituted acrylamide.
U.S. Pat. No. 4,532,048 discloses a method for inhibiting scale, such as calcium phosphate, using a 2-component copolymer of acrylic or methacrylic acid, and a lower alkenyl carboxylate or a 3-component copolymer of acrylic or methacrylic acid, a lower alkenyl carboxylate, and a lower alkenyl sulfonate or a salt thereof. A specific example of a lower alkenyl carboxylate is vinyl acetate. The hydrolysis of acrylic acid/vinyl acetate to acrylic acid/vinyl alcohol is disclosed.
Vinyl alcohol terpolymers are not obvious over terpolymers containing vinyl acetate. Polymerized vinyl alcohol units differ in structure and properties from polymerized vinyl acetate units. Polymerized vinyl acetate units may be hydrolyzed to polymerized vinyl alcohol units, however, this requires an additional step.
The record in the parent patent application points out the performance differences between vinyl acetate and vinyl alcohol. For instance, a 60/40 copolymer of acrylic acid and vinyl acetate gave a very high calcium phosphate inhibition of 90% whereas that same copolymer when hydrolyzed to acrylic acid/vinyl alcohol (weight ratio 74.6/25.4) gave 20% inhibition. Clearly, the acrylic acid/vinyl alcohol copolymer gave much lower performance than the unhydrolyzed acrylic acid/vinyl acetate copolymer. A 60/20/20 copolymer of acrylic acid/AMPS.RTM./vinyl acetate gave 91% inhibition. Yet, unexpectedly, that same copolymer when hydrolyzed to acrylic acid/AMPS/vinyl alcohol (66.5/22.2/11.3) gave 95% inhibition. In fact, in many cases, AA/AMPS/vinyl alcohol copolymers give higher performance than their parent AA/AMPS/vinyl acetate polymers. This surprising result demonstrates the non-equivalence of polymerized vinyl acetate and polymerized vinyl alcohol.
In a typical hydrolysis, a copolymer of acrylic acid and vinyl acetate in water is hydrolyzed by refluxing it for several hours with a mixture of the copolymer and sodium hydroxide. The sodium hydroxide is used in amount of equal to one mol thereof for each mol of polymerized vinyl acetate plus one mol thereof for each mol of carboxylic and sulfonic acid groups on the polymer.
This invention generally relates to inhibition of scale formation and deposition in aqueous systems. This is accomplished by adding to an aqueous system at least a threshold inhibition amount of a copolymer of at least three different monomers, i.e., monounsaturated carboxylic acid, salts and anhydrides thereof of 3 to 5 carbon atoms, an acrylamidoalkane sulfonic acid and salts thereof, and vinyl alcohol. The resulting copolymer is soluble in water and is effective in inhibiting precipitation of scale, particularly calcium scale such as calcium phosphate, calcium sulfate, and calcium carbonate. Inhibition of scale precipitation is accomplished with the herein-defined copolymers which are effective against scale in the presence or the absence of soluble metal ions such as iron, manganese, and zinc. Soluble iron ions referred to herein exist in either the II or the III oxidation state.
Other related patent applications are concurrently filed for Messrs. Amjad and Masler including one entitled "Terpolymers For Dispersing Particulates In An Aqueous Medium" and another entitled "Stabilization of Metal Ions With Copolymers Containing Acrylamidoalkane Sulfonic Acid". All of these cases disclose the use of same or similar polymers.